My First receiver:Mixer & IF

[froten]

I'm working on the new PCB, because of the limited time I hope this will not take long time.

For the time being, I would like to get more advises about the parts of the circuit such the transformers and gain, number of stages...etc.

 

[vfone]

The new wounded inductors should be fine, if the final inductance together with parallel capacitor resonate on IF frequency.
Many years ago my first receiver project was even simpler than this, but when was done I was the happiest person in the world.
Just for simplicity I would recommend to don’t use tapped inductors. The reason of tapped inductors is to don’t load the Q of the resonant circuit, decreasing in this way the overall selectivity. Anyway, lower Q means less chance to oscillate.
If somewhere in the circuit is a feedback, AGC is not helping too much for stability.

 

[G0HZU]

My initial impression having seen the coil data is that you are trying for too much gain from each stage. I wouldn't be surprised if just one stage went unstable if you played around with a reactive load at the input and output. i.e. I very much doubt each amplifer stage will be unconditionally stable.

Have you measured the gain per stage? it could be well over 30dB and it's not surprising that you are having problems. You should be able to measure this with a scope.
You should also be able to simulate this circuit on SPICE or a (non) linear simulator. My guess is it will be potentially unstable.

If you still want to keep going then add some unbypassed emitter resistance to reduce the gain. Then get the rest of the receiver working. It will still function as a receiver but it maight not have full sensitivity.

You do need to keep the taps on the transformers in order to keep the receiver fairly selective. This is because you don't have any form of ceramic IF filter to define the selectivity.
I still think you should fit a ceramic filter (for selectivity) and build the IF amplifiers with cascode FETs. This will allow a better AGC and will be more linear and more likely to be stable although you will probably only get 25dB gain per stage.

 

[froten]

A good news.

By connecting the cold end of C13, C14 & C19 directly to C4, there was big improvement in the stability. The oscillation is stopped although the circuit still has tendency to oscillate, I think there still some form of feedback caused by the layout of components. Even neutralization is making difference now and the circuit is almost under control.

I'm checking If I can still use the same layout but with double layers board and use the new layer as ground plan. May be at the components side.

Thanks guys for all help you provided.

In EPE magazine issue june 2003, a seriec of practicle radio receivers projects, which I'm thinking to build instead of designing from scratch. Some of these projects use FETS and ceramic filters.

Thanks again to everybody made any kind of help.